Various resin products are commercially available in the trade for the repair of glass cracks and, in particular, the repair of automobile windshield cracks. Such repair may be done in place without removal of the windshield and saves the cost of windshield replacement. The practically of an acceptable glass crack repair is principally due to the fact that the index of refraction of the resin is substantially the same as that of the glass.
A crack in glass is visible because as light rays pass the boundary between the glass and the air in the crack they are bent due to the difference in the speed of light in glass as compared with the speed in air. Light that is reflected back from such boundaries comes at a different angle than if there were no crack and hence the eyes and brain of the observer sense the discontinuity. Index of refraction for any material is a ratio of the speed of light in that material compared to the speed in a vacuum. If two materials have the same index of refraction there will be no bending of the light rays at a boundary between the materials and an observer will not sense the discontinuity. Thus a crack filled with such a resin will essentially seem to disappear.
There are different formulations of resins. Some are aerobic or "air drying" and others are cured or hardened by radiating them with ultraviolet light which is usually provided by a lamp made for this purpose. The aerobic resins come in two parts and are mixed before use. Their curing begins from the time of mixing whereas the cure of the ultraviolet types begins when they are subjected to the ultraviolet light.
Automobile windshields are typically made of a "sandwich" of two layers of glass laminated to an inner layer of resin material such as polybutyral.
The windshield has a periphery which defines a glass area withing the periphery for each glass layer. The glass area for each layer has an outer exposed surface and an inner surface in contact with the inner plastic (polybutyral) layer. Also, for each glass layer there is an edge defined by the periphery extending between the outer surface and the inner surface joining these surfaces at an angle such as 90.degree. to form a corner. Many types of breaks may occur in such windshields. One, which is usually caused by the impact of a rock, is called a "bullseye". A bullseye is a type of a stone break which has a circular appearance. It often has a cone shaped piece of glass detached from the outer layer. The apex of the cone faces the outer surface and the base of the cone may be forced partly into the inner layer. The apex may or may not connect to the outer atmosphere.
Sometimes the impact of a rock or other object will result in small radiating cracks from the impact point and is then called a "star break". A combination of a bullseye and a star, called a "combination break", can also occur. Impacts often cause linear cracks in windshields. They may originate at a stone break or may, instead, originate at a point. They may extend to the glass edge or they may terminated at another point.
Various tools have been proposed to aid in filling stone breaks with resin. One is described in U.S. Pat. No. 3,993,520 to Werner and utilizes an injector assembly having a piston inside a cylinder. When the piston is depressed the resin is injected out the open end of the cylinder and into the stone break. The assembly is held to the surface of the glass by suction cups.
A similar apparatus with an adjustable arm is disclosed in U.S. Pat. No. 4,291,866 to Petersen. A variation of the Werner apparatus is in U.S. Pat. No. 4,569,808 to Smali while U.S. Pat. No. 4,744,841 to Thomas utilizes vibration and heat. An apparatus with a spring loaded air exit is disclosed in U.S. Pat. No. 4,775,305 to Alexander and U.S. Pat. No. 4,814,185 to Jones has a side tube for introduction of the resin. The use of vacuum to aid the resin injection is explored in U.S. Pat. No. 4,820,148 to Anderson and U.S. Pat. No. 4,919,602 to Janszen. Other related patents are U.S. Pat. No. 4,419,305 to Matles and U.S. Pat. No. 4,385,879 to Wilkinson.
As far as is known, the background art discussed above was designed for stone breaks rather than long cracks (over six inches).
Short cracks are seen as small dots on the surface of the glass, or as a bullseye, a star-break, a combination bullseye and star break, and sometimes as variants on these configurations. It is well known that stone breaks are "unsurfaced" that is they extend below the surface of the glass; or if they do extend to the surface they are so tight that they must be treated as unsurfaced when being repaired. Thus the prior repair methods and equipment employ application of vacuum at an entry location which is the impact spot which caused the damage, followed by injection of resin under pressure to flow into the crack. This is called "stone damage art". In this manner the resin replaces the air in the crack. However, stone damage art cannot be used to repair cracks which are "surfaced", that is where the crack is open at the surface of the glass. Such cracks have been consistently referred to in the art as over 6 inches in length. There has been an inability to effectively repair cracks over 6 inches, and a long felt need for a method and equipment to do so. Stone damage art employs resin viscosity normally in the range of 10-30 c.p.s., but not exceeding about 50 c.p.s. In general the viscosity must be low enough to easily flow in the tight cracks around a stone break. Thus a thin watery viscosity has been desired.
Most such long cracks have an end which terminate at a location on the glass area which is neither the edge nor the point of impact. This termination location is called a "point". The point itself is very tight and locally may be unsurfaced. Further most such long cracks will extend from a point to the edge of the glass with an impact location intermediate these ends. Other such long cracks will radiate from an impact location to the glass edge or to a point. Frequently the crack will extend in two opposite directions from an impact point, ending in a point in one direction and at an edge in the other direction. Some cracks terminate at a point at each end. Sometimes a Y formation will appear. A surfaced crack will usually be unsurfaced immediately proximate a point.